Refinery processes such as hydrocracking, coking and catalytic cracking produce substantial quantities of sour gases which are typically collected in a gas plant such as an unsaturated gas plant (USGP). FIG. 1 presents a typical process schematic for a fluid catalytic cracking plant incorporating USGP and illustrating the particular unsaturated gas plant downstream of the main fluid catalytic cracking (FCC) fractionator. In the FCC process, feed, such as gas oil (20), is heated in a heater (21) and passed to the bottom of the riser section (22) of the catalytic cracking reactor (23) where it comes in contact with the catalyst recirculated from catalyst regenerator (24) at high temperature, where it is cracked to provide an overhead stream (25) while catalyst is recycled through conduits (26,27). The overhead product, after catalyst separation, passes to the main FCC fractionator (19) where it is separated into a gasoline and light gases fraction (18), light fuel oil (28) and heavy fuel oil (29) and a bottoms fraction (30) which may be recycled (31) to the FCC feed. Overhead (18) passes to an accumulator (17) to provide a reflux stream (16) to the main fractionator. Also from the accumulator wet gases are compressed in compressors (15), cooled (31), and passed (32) to a lower section of an absorber/stripper (33). A "wild" or unstabilized gasoline fraction from the accumulator is passed to an upper portion of the absorber/stripper through conduit (34). The absorber/stripper overhead product is light gases and C.sub.2 - fraction (35); these are passed sponge to absorber 45 for separation of the C.sub.2 - fraction as overhead 46. The bottom fraction is passed (36) to debutanizer (37), where a C.sub.4 - overhead is passed to depropanizer (38) to provide a propane and propylene product (39). The debutanizer bottoms effluent comprises a C.sub.5 + fraction and is passed through conduit (40) to gasoline splitter (41) to produce a light gasoline fraction (42) and heavy gasoline fraction (43), a portion of which is returned (44) to absorber/stripper (33).
In so far as olefinic gases produced in the typical refinery operations described above generally contain acid gases their removal is conventionally conducted as part of the USGP operations. These acid gases are primarily hydrogen sulfide and carbon dioxide but also include hydrogen cyanide. A large variety of acid gas removal processes are available that separate into processes based upon chemical solvent action including solvents such as monoetholamine (MEA), diethyanolamine (DEA), and hot potassium carbonate; processes depending on physical solvent action such as Selexol, Rectisol, etc.; and processes based on dry adsorbents such as molecular sieves, activated charcoal, iron sponge and the like. Conventionally, in the prior art these acid gas removal processes are installed downstream of the sponge absorber and debutanizer. Consequently, the acid gases are carried through the various upstream separation processes of the USGP including the absorber-deethanizer, sponge absorber and debutanizer. This configuration tends to increase the rate of acid gas induced corrosion of a large portion of the vessels and ancillary equipment in the USGP, leading to increased maintenance operations and plant downtime.
It is an object of the present invention to provide a process for the operation of an USGP in a manner which reduces the corrosive effect of acid gases on plant operations.
Another object of the present invention is to provide a process for the removal of acid gases in the process stream to USGP upstream of USGP gas separation operations.
Yet another object of the present invention is to provide a unique acid gas absorber apparatus design for high recovery USGP operations.
Another objective is to unload or reduce the required total throughput of the unsaturated gas plant.